Replying to Need advice: upgrading my Subaru Outback

I've owned a 1999 Subaru Legacy Outback for a few months now and I'm interested in tinkering around with it and making some performance upgrades. Right now it has really sluggish acceleration (0-60 in 13 seconds) and a top speed around 120 mph, but corners really well. Where would be a sensible, financially sensitive place to start to get a bit more vvvrrrooom out of it? I think I'd like to improve the acceleration most of all, if that's possible. I drive off road almost as often as on road, and I have no interest in cosmetic changes. I also don't know a whole lot about cars so bear with me if I ask stupid questions.